The efficiency of a boiler increases with reducing outlet temperature of its combustion products, i.e. when less heat is carried away by its flue gases. The combustion products contain carbon dioxide and water. If the temperature of the combustion products is reduced far enough (below their dew point temperature, i.e. about 55.degree. C., a portion of the water which they contain condenses, thereby releasing recoverable latent heat. Whence the term "condensation boiler" for designating such very high efficiency equipment which recovers as much of the sensible heat in the flue gases as possible together with a portion of the latent heat corresponding to condensation of the combustion-produced water vapor. This result is obtained by increasing the area of the heat exchanger between the flue gases and the water to be heated, or by providing a second or recovery heat exchanger in addition to the main heat exchanger.
In a condensation boiler the combustion products are nevertheless exhausted from the boiler at a temperature such that a portion of the total theoretically recoverable energy is lost. In addition, since the combustion products have been cooled down to saturation when they are rejected into the atmosphere, their rejection into a relatively cold atmosphere gives rise to a wreath of vapor which is several meters long and which is particularly unattractive when the flue outlet point is situated on the wall of a residential building, for example.
Proposals have already been made, for example in French patent application No. 8315657 to implement means which tend to limit the appearance of a wreath when the combustion products from a condensation boiler are rejected into the atmosphere. In this prior system, the combustion products are diluted with a fraction of the air taken from the immediate vicinity of the suction inlet for oxygen-carrying air and for rejecting combustion products into the ambient air. Such a system for exhausting the combustion products from a condensation boiler into the ambient air serves to reduce the wreath of vapor to a certain extent, but requires a heat exchanger to be disposed in the immediate vicinity of the combustion product outlet, and this is not always convenient nor is it always favorable for optimum heat recovery.
Preferred embodiments of the present invention seek to reduce the risk of a wreath of vapor appearing at the outlet from the exhaust duct for the combustion products from a condensation boiler to as small a value as possible, while reducing unwanted exchanges of heat within the condensation boiler as much as possible.